1. Field of the Invention
The present invention relates to a highly reliable semiconductor laser with a low noise and low power consumption, and to an optical device with employment of such a semiconductor laser.
2. Description of the Related Art
Semiconductor lasers have been used as light sources for an optical disk recording/reproducing apparatus and the like. When light reflected from an optical disk is fed back to a semiconductor laser, this reflection light is optically coupled to a light resonator fabricated outside the semiconductor laser. Thus, the oscillation longitudinal mode of the semiconductor laser is varied under influences of the unstable resonator formed outside the laser, so that relative intensity noise (abbreviated as "RIN") of the laser light would be increased due to a so-called "mode hopping".
As the waveguide mechanisms of the conventionally utilized semiconductor lasers, there are a gain guide type waveguide and a refractive-index guide type waveguide. Referring to FIG. 1, an example of the gain guide type semiconductor laser will now be explained. In the semiconductor laser of FIG. 1, reference numeral 21 indicates a substrate made of n-type GaAs and the like, reference numeral 22 shows a cladding layer made of AlGaAs and so on, reference numeral 23 denotes an activating layer made of undoped AlGaAs and the like, and reference numeral 24 represents a cladding layer made of p-type AlGaAs and the like. Further, reference numeral 25 shows a capping layer made of p-type GaAs and the like, reference numeral 26 represents a high resistance region formed by way of ion implantation, and also reference numerals 27 and 28 are ohmic electrodes.
In this case, the oscillation longitudinal mode is a multimode oscillation as represented in FIG. 2. Coherence of the laser light is lowered due to this multimode oscillation, and coupling of the laser light with the external resonator becomes weak. Therefore, noise may be reduced. Since the threshold level of this gain guide type semiconductor laser is high, power consumption thereof is increased. The gain guide type semiconductor laser has such a drawback that since the cophasal surface of the laser light is curved astigmatism may occur, whereby the laser light cannot be focused onto an optical disk by employing a single spherical lens.
On the other hand, there is shown an example of the refractive-index guide type semiconductor laser in FIG. 3. In this refractive-index guide type semiconductor laser of FIG. 3, a cladding layer 22 made of n-type AlGaAs or the like, an activating layer 23 made of undoped AlGaAs or the like, and another cladding layer 24A made of p-type AlGaAs or the like are laminated on a semiconductor substrate 21 made of n-type GaAs or the like. Furthermore, a current stopping layer 29 made of n-type GaAs and the like is formed on this cladding layer 24A in a predetermined pattern. Subsequently, this current stopping layer 29 is covered on its whole surface with another cladding layer 24B made of a p-type AlGaAs or the like and another capping layer 25 of p-type GaAs or the like. Thus, both sides of the light emitting region are defined by the current stopping layer 29, thereby lowering the threshold level. In FIG. 3, reference numerals 27 and 28 indicate electrodes, respectively.
As described above, the refractive-index guide type semiconductor laser is featured such that the threshold level thereof becomes low, the low power consumption is realized, and astigmatism becomes small. The oscillation longitudinal mode of such a refractive-index guide type semiconductor laser corresponds to a single oscillation longitudinal mode as shown in FIG. 4. Consequently, this type of semiconductor laser has such a drawback that optical feedback induced (pumped) noise becomes large.
To reduce, or suppress the optical feedback induced noise of the above-described semiconductor laser, perfect optical isolation should be implemented. However, such a perfect optical isolation is practically difficult, and also very expensive optical components are necessarily required. As a consequence, the refractive-index guide type semiconductor laser per se must own such a characteristic that the semiconductor laser cannot be readily mode-pumped even under presence of returning light.
On the other hand, in a semiconductor laser where the activating layer as shown in either FIG. 1 or FIG. 3 is extended along the horizontal direction, if reflectivity of the facet thereof on the side of the light output port would be relatively lowered so as to satisfy the noise characteristic for such optical feedback, loss in the optical resonator would become large and threshold carrier density would be increased. As a result, deterioration of the activating layer would be rapidly emphasized and, thus, there is a risk that high reliability could not be achieved.